John McCARREY (USA)
Dr. McCarrey received his Bachelors degree in Animal Science and his Masters and PhD degrees in Genetics, all from the University of California, Davis. He did a postdoctoral fellowship with Dr. Susumu Ohno at the City of Hope in Duarte, California. He then joined the faculty in Reproductive Biology at the Johns Hopkins School of Public Health. In 1991 he moved to the Department of Genetics at the Southwest Foundation for Biomedical Research in San Antonio, Texas, and in 2001 he assumed his present position as Professor of Cell & Molecular Biology at the University of Texas at San Antonio. He holds joint appointments in the Departments of Obstetrics and Gynecology and Cellular & Structural Biology at the University of Texas Health Science Center at San Antonio, and in the Department of Comparative Medicine at the Southwest Foundation for Biomedical Research. He is also an affiliate scientist of the Southwest National Primate Research Center in San Antonio, and Director of the San Antonio Cellular Therapeutics Institute. In 2012, Dr. McCarrey was named the Robert and Helen Kleberg Distinguished Chair in Cellular & Molecular Biology.
Dr. McCarrey’s research interests focus on the development, differentiation and function of mammalian germ cells and stem cells. He discovered the first example of a functional, germ-cell-specific retroposon in the human genome. He has published several papers on mechanisms that regulate germ-cell-specific gene expression in mammals. He has also published several papers on mechanisms of epigenetic programming that function during germ cell development and gametogenesis, and in stem cells. Additional research interests include mechanisms governing X-chromosome activity in germ cells and early embryos, mechanisms governing genetic integrity in germ cells and stem cells, the effects of cloning and assisted reproductive technologies on genetic integrity, and the development of nonhuman primate model systems for studies of stem cell research and regenerative medicine. Recently he has focused on mechanisms involved in the induction of epimutations by environmental disruptions including the use of assisted reproductive technologies, and the extent to which these are reprogrammed in the mammalian germ line. His newest interest is in the development of foundational spermatogonial stem cells in the mammalian testis.
Abstract
On the Origin of Spermatogonial Stem Cells
Spermatogonial stem cells (SSCs) sustain steady-state spermatogenesis in the adult testis. SSCs, which are the only self-renewing cell type in the spermatogenic lineage, are a subset of undifferentiated spermatogonia that derive from prospermatogonia and give rise to progenitor spermatogonia that can then initiate spermatogenic differentiation. Multiple theories abound regarding the dynamics by which SSCs are initially specified in the immature testis and/or maintain their fate in the adult testis. One theory holds that SSC fate is transient and can be adopted or lost by individual spermatogonia almost at random1. This theory predicts that the initial “foundational” SSCs found in the immature testis represent a subset of undifferentiated spermatogonia that are not fundamentally distinct from other undifferentiated spermatogonia. An alternative hypothesis holds that foundational SSCs derive from a specialized subset of developing prospermatogonia that are predetermined to adopt and maintain the SSC fate2. In the mouse, functional SSCs can be distinguished on the basis of a transplantation assay originally developed by Brinster et al.3. We reasoned that if specification of foundational SSCs in the immature mouse testis results from a distinct, prospectively acting epigenetic program, this program should be uniquely evident in transplantable SSCs. Testable predictions of this hypothesis are that transplantable SSCs should display a gene expression pattern that is consistently distinguishable from that detected among other undifferentiated spermatogonia, and that genes that are differentially expressed in SSC and non-SSC spermatogonia, respectively, should be accompanied by similarly distinguishable profiles of epigenetic programming.
The Id4-egfp transgene marks transplantable SSCs4, and selective recovery of the brightest and dimmest thirds, respectively, of Id4-eGFP+ spermatogonia yields SSC-enriched and SSC-depleted subpopulations5. Bulk and single-cell RNA-seq have shown that these spermatogonial subpopulations express distinct transcriptomes5-7. We used multi-parametric integrative analysis of genome-wide patterns of DNA methylation, six different histone modifications (H3K4me1/2/3, H3K9me3, H3K27me3, H3K27ac), and chromatin accessibility, plus spermatogonial subtype-specific bulk and single-cell transcriptome data to generate the first genome-wide annotation of epigenetic profiles in transplantation-validated SSCs. Our data show that patterns of differential gene expression and associated active promoters and enhancers consistently distinguish ID4-EGFPbright/SSC-enriched and ID4-EGFPdim/SSC-depleted spermatogonial subtypes. We next examined motif enrichment within differentially active promoters and enhancers in each subtype to predict major regulators of SSC fate. We are now using immunohistochemistry and gene- and cell-type specific ChIP to test candidates for factors differentially bound to promoters and enhancers in ID4-EGFPbright and ID4-EGFPdim spermatogonia at P6, including TC4F, BCL11a, & MAZ, and LHX1, DMRT1, & FOXP1, respectively. Taken together, our findings suggest that these factors signal a unique epigenetic landscape that directs unique gene expression patterns required for specification of SSC fate, and that foundational SSCs are therefore fundamentally distinct from other undifferentiated spermatogonia. This, in turn, is consistent with our hypothesis that foundational SSCs derive from an epigenetically predetermined subset of prospermatogonia.
1 Hara et al. [Yoshida lab] (2014) Cell Stem Cell 14:658-672.
2 Murphey et al. [McCarrey lab] (2013) Biol Reprod 88:6. doi: 10.1095/biolreprod.112.103481.
3 Brinster & Zimmermann [Brinster lab] (1994) Proc Natl Acad Sci (USA) 91:11298-302.
4 Chan et al. [Oatley lab] (2014) Genes Dev 28:1351-1362.
5 Helsel et al. [Oatley lab] (2017) Development 144:624-634.
6 Hermann et al., [Hermann & McCarrey labs] (2015) Biol Reprod 92:54. doi:10.1095/biolreprod.114.125757.
7 Hermann et al., [Hermann & McCarrey labs] (2018) Cell Reports, In press.
Supported by the Kleberg Foundation, the Hurd Foundation and the NICHD (HD078679)